Spotlights and other optical devices, such as telescopes and microscopes, contain an optical tube with a lens or a lens system of multiple lenses, which are either fixed in the tube or movably disposed therein. For optical reasons it can be necessary not no move the lens or the lens system within the tube for focusing and defocussing, but to move the tube together with the fixed installed lens or the lens system relative to the housing of the Spotlight or optical device. This is especially true if the edge of the tube would interfere with the optical path by partially blocking the same when adjusting the lens or the lens system within the tube. For example, microscope objectives and a number of photographic objectives are assembled according to this principle of construction.
In order to ensure exact adjustment of the Spotlight or optical device, accurate guidance and movement of the tube relative to the housing accommodating the tube is necessary. Two basic embodiments are known for this purpose.
In a first embodiment a screw thread is provided for adjusting the tube, whereby the housing is provided with an internal thread, whereas the optical tube comprises an external thread that matches the internal thread. An adjustment of the tube along the optical axis of the Spotlight, or optical device takes place by rotating the tube relative to the housing, wherein the pitch of the screw thread determines the displacement of the tube during the adjustment.
With this embodiment it is disadvantageous for one thing that a large number of rotations are necessary in order to displace the tube by a certain amount, and for another thing that because of the use of a screw mechanism the tube and the housing must have a circular outer cross-section or inner cross-section.
In order to enable any cross-sectional shape for the tube and the housing, e.g. a square tube and a square housing, in a second embodiment the adjustment of the tube takes place by means of a linear drive, with which the tube can be displaced along the optical axis of the Spotlight or optical device without twisting relative to the housing. Such a linear drive consists e.g. of a toothed rack connected to the tube or formed on the outer surface of the tube, which is driven by a pinion that is connected to a rotation axle, which comprises a rotary knob for manual operation and adjustment of the tube. This principle of construction is e.g. used for simple microscopes.
It is disadvantageous with this embodiment that depending on the inclination of the tube and of the housing a variable weight acts on the tube, which can cause the tube to move away from the selected adjustment into the housing or out of the housing if the toothed rack transmission has no or too little self-locking.
This is especially the case for the use of a linear drive for linear tube adjustment for a powerful Spotlight, e.g. for displacing a Fresnel lens relative to a light, generation unit. Here such heavy weights occur that a high degree of self-locking of the linear drive is necessary. However, a high degree of self-locking of the linear drive causes an increase of the force required to adjust the tube, whereby the operation for adjustment of the tube is made considerably more difficult. Because powerful spotlights generally have a large diameter and are associated with heavy accessories, such as e.g. metal swing gates and glass diffuser disks, which have to be moved into each operating position, the use of a simple rack and pinion gear for tube adjustment is excluded, because the forces required for operating the spotlight are too large.